I. Field of the Invention
The present invention relates to the fields of pharmaceuticals, medicine and cell biology. More specifically, it relates to pharmaceutical agents (compounds) which are useful as integrin receptor antagonists, with particularly exceptional biological activity as antagonists of a group of integrins that mediate the pathologic processes of angiogenesis and fibrosis. As such, these compounds are useful in pharmaceutical compositions and in methods for treating conditions mediated by such integrins by inhibiting or antagonizing these integrins.
II. Description of Related Art
Integrins are a family of integral cytoplasmic membrane proteins that mediate cell interactions with other cells and with the extracellular matrix. Approximately one third of the members of the integrin family directly bind to a specific amino acid motif, arginine-glycine-asparate (RGD), that is contained within the sequence of their cognate protein ligands. It has been established in the art that peptides containing the RGD sequence, and synthetic small molecule compounds that mimic the RGD sequence, are capable of binding to these integrin receptors with varying degrees of specificity, and thereby inhibit the binding to normal physiologic ligands (Millard et al., 2011.). The biological effects of treatment with such agents is dependent on intrinsic molecular properties, reflected in the structure, that determine to what degree a particular integrin, or combination of integrins, is inhibited in a body tissue over a period of time.
Many human diseases are characterized by either or both of two common contributing pathological mechanisms: angiogenesis and fibrosis. Different subsets of the RGD-binding integrins have predominant roles in driving these dual processes, so that simultaneous antagonism of angiogenesis and fibrosis requires agents capable of binding potently to several target integrins. This contrasts with agents designed specifically for binding to a single integrin which may be less effective in some applications due to their more restricted mechanism of action.
Integrins which have been shown to have a role in promoting angiogenesis include, αvβ3, αvβ5, and α5β1. αvβ3 and αvβ5 were initially described as mediators of bFGF- and VEGF-induced angiogenesis, respectively, in corneal or choriallantoic models. More recently, data from studies using mice lacking these integrins also support an important functional role for α5β1. The integrin α5β1 (also known as VLA-5) is often referred to as the ‘classic fibronectin receptor’ reflecting its well characterized interaction with this extracellular matrix protein. Cells expressing α5β1 bind to fibronectin in a region that incorporates the ninth and tenth type III fibronectin repeats, the latter of which contains the RGD motif critical for integrin binding. In addition to fibronectin, α5β1 has been reported to interact with other RGD-containing extracellular matrix proteins including fibrinogen, denatured collagen, and fibrillin-1 (Bax et al., 2003; Perdih, 2010; Suchiro et al., 2000). These ligands are components of the provisional matrix that is laid down by cells as part of the wound healing response in tissues. Key components of this response are angiogenesis (new blood vessel formation) and fibrosis (scar formation) which are beneficial for healing of acute injuries, but can be deleterious in many disease contexts.
Antagonists of RGD-binding integrins should be useful for treatment of human diseases having angiogenesis or fibrosis as a principal part of their pathology. In particular, the important role of α5β1 in angiogenesis is supported by numerous studies. For example, mice lacking this integrin exhibit embryonic lethality at day 10-11 with a phenotype that includes defects in both the embryonic and extraembryonic vasculature (Yang et al., 1993). Angiogenic cytokines such as bFGF, IL-8, TGFβ, and TNFα upregulate α5β1 expression on endothelial cells in vitro and in vivo, and immunohistochemistry shows coordinated increases in both α5β1 and fibronectin staining in blood vessels from various types of human tumor biopsies and xenograft tumors in animals (Collo, 1999; Kim et al., 2000). Monoclonal antibodies that specifically inhibit α5β1, and compounds that have been described as α5β1 inhibitors, significantly reduce angiogenesis in a number of experimental models (Kim et al., 2000; Bhaskar et al., 2007; Livant et al., 2000; Zahn et al., 2009).
Because α5β1 expression is not confined to the endothelium, it has other functional roles in addition to angiogenesis. It is expressed to varying degrees in many cell types including fibroblasts, hematopoietic and immune cells, smooth muscle cells, epithelial cells, and tumor cells. Expression on tumor cells has been implicated in the progression of tumor growth and metastasis (Adachi et al., 2000; Blasé et al., 1995; Danen et al., 1994; Edward, 1995). In human fibroblasts, α5β1 promotes motility and survival (Lobert et al., 2010). In pancreatic stellate cells, it interacts with connective tissue growth factor to stimulate adhesion, migration, and fibrogenesis (Gao and Brigstock, 2006). It has been shown that pharmacologic antagonism of α5β1 inhibits the attachment migration, and proliferation of human retinal epithelial cells in vitro, and reduces retinal cell proliferation and scarring when administered intravitreally to rabbits with retinal detachment (Li et al., 2009; Zahn et al., 2010).
Multiple RGD-binding integrins of the alpha v family have been implicated in promoting the biological activation of the latent pro-fibrotic cytokine TGFβ. This is mediated by binding to the latency associated peptide (LAP), particularly by αvβ6 and αvβ8, but also by αvβ1, αvβ3, and αvβ5. These integrin interactions are all critically dependent upon the amino acid sequence arg-gly-asp (RGD) contained in LAP. Indeed, mice containing a mutation in the RGD sequence are incapable of cytokine activation and phenocopy TGFβ-null mice. It is anticipated that simultaneous inhibition of multiple integrins with the potential to activate TGFβ may have particular utility to prevent or treat a range of fibrotic conditions. In addition, such broad spectrum integrin antagonists may be particularly useful for simultaneous modulation of both angiogenesis and fibrosis.